A number of active vehicle suspension systems have been proposed which actively control the relative movement between the wheel and the vehicle body by using a linear hydraulic actuator according to the road condition and the behavior of the vehicle body. One of such active vehicle suspension systems is disclosed in Japanese patent laid open publication (kokai) No. 62-289420 in which an actuator is provided in parallel with a conventional suspension spring consisting of a coil spring so that the vibrations of the unsprung mass may be absorbed by the suspension spring while the vibrations of the sprung mass are absorbed by the actuator.
In such a vehicle suspension system, the deformation of the coil spring obviously affects the movement of the sprung mass or the vehicle body. Further, the damping force due to the resistance to the flow of the hydraulic fluid in the hydraulic actuator, in particular the flow resistance at the hydraulic control valve adversely affects the capability of the hydraulic actuator to absorb the vibrations of the sprung mass.
As a control method for reducing the vibrations of the sprung mass due to such interferences, there is known the sky hook damper control in which the hydraulic damper produces a force proportional to the absolute vertical velocity of the sprung mass so that a hypothetical damper may be provided between a fixed point in the space and the sprung mass or the vehicle body. In this case, it is difficult to directly detect the absolute velocity of the vehicle body, and the actuator has to be controlled, in reality, according to a value obtained by integrating the vertical acceleration of the vehicle body. However, in a low frequency range of 1 Hz or lower, since the integrated value has a tendency to drift, the sky hook control becomes impractical. Therefore, in practice, the actuator is controlled according to a signal obtained by filtering the acceleration with a first-order delay circuit which can be expressed by a transfer function 1/(1+Ts) where T is a time constant. Therefore, the sky hook damper control does not exist in a low frequency range. If the time constant is increased in value to extend the sky hook damper control to a lower frequency range, the control action becomes increasingly unstable.